Discovering plant galls formed by insects

Gall: “An abnormal growth of plant tissue produced by a stimulus external to the plant itself.” – S.W. Frost in Insect Life

Galls are common formations on plants, usually caused by insects (but sometimes by a fungus or some other non-insect life form). Galls can form on roots, stems and twigs, leaves, flowerheads and buds. They can be smooth, rough, hairy, spongy, hard, soft, spiked. They can be round, oval, spindle-shaped, and even look like flowers. They may not look like any of these. They may, instead, look like a black spot, a leaf blister, or may be mistaken for clusters of insect eggs. Their colour often changes. Goldenrod galls are green when the plant is living, but when the plant dies back, the galls take on the brown hue of the stem, although before they do so, they may also appear burgundy or bronzed. Some galls are vividly red, others may be two-toned, or patterned, but most are, not surprisingly, green or brown. Some appear singly, others in clusters. And one plant may have several different types of galls, made by insects of different families. Some galls are strikingly noticeable, while others are found only by dint of looking. Just to confuse things, not all swellings on plants are galls, as some are caused by leaf miners and stem borers.

If you want to identify the gall-makers, you need to know your plants, because many of the insects are very host-specific. And not only that, but most insects will only form galls on certain parts of the plants. Thus, you may find a plant with galls on the leaves and on the stem or twigs, but different species will be responsible! Furthermore, the location is an important aid to sorting out who might have made the gall. So take your notebook and make notes when you want to identify possible gall-makers. Some plants seem to attract a variety of gall-making insects. Chief among these are willows (Salix), oaks (Quercus), and goldenrod (Solidago).

Gall-making insects may be flies (e.g., midges, other Cecidomyiids, and fruit flies), bugs (e.g., aphids, Phylloxera), sawflies, wasps (mostly Cynipidae), moths (mostly Tortricidae), nematodes, and even beetles. Members of the Arachnidae also get into the act, with gall mites (Eriophyidae) responsible for many galls. However, the gall midges in the family Cecidomyiidae, sub-family Cecidomyiinae, and Cynipid wasps, are probably the most common gall-makers. The photos below show some of the different types of galls, made by various insects.

One way to determine who has made a gall is to bring the galls home and rear them, which means putting them in an enclosed container (a jar with a mesh cover, for example) and see what emerges. However, be warned, that many parasites attack gall larvae and so what comes out may not be the original gall inhabitant. This just makes it all the more fascinating. Because most gall-making insects are tiny and rarely seen, rearing them may be your only chance to see them. In nature, the galls are far more obvious than the insects.

Why do galls form, and for what purpose? The short answer is that galls are protective coverings for the egg and later the larva, inside. They also provide food for the developing larva. Larvae either spend the winter in their galls or, in a number of cases, emerge, drop to the ground and pupate in the soil.

Galls are formed when still-growing plant tissue is invaded by an insect (or fungus). The plant grows around the foreign object to form the shapes we know as galls.

When galls are very noticeable, as with the grape leaf galls that distort the leaves, it is thought that the plants themselves are diseased or damaged, but they are not. In most cases, galls do no major damage to the plants.

They do, however, in some cases, provide food for other wildlife. The goldenrod galls are best known in this regard, for many of us have observed woodpeckers or chickadees pecking away at the galls to extract the juicy larva inside. Squirrels are also fans of goldenrod galls. If you look at a stand of goldenrod and examine the abundant galls, you’ll see that many have been preyed on. Sometimes it can be difficult to find an unopened goldenrod gall.

And finally, galls are not just fascinating formations there to intrigue us. Over the centuries they have been used by humans as medicine, food, and for the production of dyes and inks.

Summer is when galls are most abundant and diverse. Because so many galls are formed on leaves, once winter comes, they disappear. However, winter is the best time to look for the bigger galls on twigs and stems. Just check out shrub willows, goldenrods, ash trees and oak trees and see what you find.

This is just a brief introduction to galls, which I have been fascinated with for years. I hope you will also find them interesting and begin your own study of them.